Hitherto, a flow passage structure has been known as a structure for mixing plural fluids so as to cause an interaction therebetween. The flow passage structure is used in, for example, a reaction device that obtains a desired reaction product by causing a chemical reaction between fluids as the interaction of plural reactant fluids. An example of the flow passage structure is disclosed in Patent Document 1 below.
Patent Document 1 below discloses the reaction device that uses a flow passage structure having plural circulation paths formed therein. Each circulation path includes a first introduction path into which a first reactant fluid is introduced, a second introduction path into which a second reactant fluid is introduced, a merging path which is connected to both downstream sides of both introduction paths and merges and mixes reactant fluids flowing through the respective introduction paths, and a reaction path which is connected to the downstream side of the merging path and causes a reaction between the reactants included in the fluids while circulating the fluids merged in the merging path. The flow passage structure is formed by laminating plural plates. The first introduction path and the second introduction path are formed in a front surface of one plate, and the reaction path is formed in a rear surface of the plate. The merging path connects the ends opposite to the reactant introduction sides of the first introduction path and the second introduction path and penetrates the plate from the front surface toward the rear surface so as to connect the ends to the end of the reaction path. Further, the reaction path is formed in a tortuous shape of which the flow passage length increases in order to promote the reaction between the reactants by increasing the merged reactant staying time. Specifically, the reaction path has a shape which is formed by alternately connecting a portion formed in the rear surface of the plate and extending toward one side of the plate in the width direction and a portion extending toward the opposite side to the one side at the downstream side of the portion.
However, in the flow passage structure of the related art, since the reaction path is formed in the above-described shape, it is difficult to clean the inside of the reaction path. Specifically, in a case where the reaction path is formed in the above-described shape, even when a cleaning unit is inserted from one end of the reaction path into the reaction path, the cleaning unit may not easily reach the deep inner portion thereof. For this reason, for example, when precipitate or foreign substance blocks the inside of the reaction path, the precipitate or foreign substance may not be easily removed. In this way, in the above-described flow passage structure, there are problems in which the inside of the reaction path may not be sufficiently cleaned and a considerable time is spent for cleaning operation.